From Nature:
Find it hard to identify the smell? A teaching session backed by electric shocks might teach you to learn the difference. Wen Li, at Northwestern University in Evanston, Illinois, and her colleagues, have shown that the brain can be shocked into sniffing out the differences between molecules that they would not usually be able to distinguish by smell. The result confirms that ‘fear conditioning’ can be used to awaken the highest capacity of human abilities, and shows that people can spot smell differences once thought perceivable only to other animals.
Li works in the neurology lab of Jay Gottfried, which has a history of investigating the sense of smell and how this affects perception. In this experiment, they looked at people’s ability to smell the difference between enantiomers — non-superimposable mirror images of molecules. Although rats can usually tell these molecules apart, humans can’t always do so. The molecule carvone, for example, smells to people like caraway when in one form, and like spearmint when in its mirror image. But most enantiomers smell identical to people.
No one knows why humans’ sense of smell differs so much from rats’. Some suggest that modern humans have lost a fine-tuned sense of smell as this sense is not generally needed for day-to-day survival. So the team set out to see whether humans really are missing something, or whether they simply aren’t trying hard enough.
More here.